For the
third time in two days, NASA postponed the launch of its asteroid-bound Dawn
probe Saturday, with liftoff now slated for no earlier than September.

The new
delay for Dawn comes after its planned Monday liftoff was moved to
July 15 earlier Saturday, but now clears the spacecraft?s Cape Canaveral
Air Force Station launch site for the August flight of NASA?s
Mars Phoenix lander.

NASA
spokesperson George Diller said mission managers opted for the new launch
target because of limited liftoff opportunities in July and the need to prepare
for the planned Aug. 3 Phoenix flight.

?A September
launch for Dawn maintains all of the science mission goals that a July launch
would have performed,? Diller said in an update from NASA?s Kennedy Space
Center in Cape Canaveral.

The $449
million Dawn mission will be the first to launch a probe to both
Vesta and Ceres, the two largest space rocks in the solar system, in the
Asteroid Belt between the orbits of Mars and Jupiter.

The eight-year
mission is slated to carry Dawn first to Vesta in October 2011, where it will
orbit the bright, dense space rock. After a detailed mapping mission, Dawn is
slated to move on to spherical Ceres - which is also considered a dwarf planet
- by February 2015.

NASA
initially had until July 19 to launch Dawn before standing down to allow
preparations for Phoenix?s August launch.

Dawn?s
mission has weathered a series of
delays over the last week. Plans for a Friday launch were reset to Sunday
after nearby storms prevented fueling of the spacecraft?s Delta 2 rocket.
Mechanical issues with a telemetry relay aircraft and the unavailability of a
tracking ship delayed the planned launch from Sunday to Monday, and ultimately
to July 15.

NASA has
until about Oct. 20 to launch Dawn, after which the Vesta and Ceres begin to
move away from each other in their respective orbits, the mission?s principal
investigator Chris Russell has said. They two space rocks will be near each
other again in about 15 years, he added.